The present invention relates to distributed feedback (DFB) lasers, and more particularly to tunable DFB lasers.
Distributed feedback (DFB) laser arrays with multiple DFB laser diodes are coupled through a multimode interference coupler to provide a single output. The DFB laser array is temperature tuned to adjust the wavelength that is output by the DFB laser diodes. For example, if each DFB laser diode provides 3 nanometers (nm) of temperature tuning, a DFB laser array with four DFB diode lasers covers 12 nm, which is equivalent to sixteen 100 Giga Hertz (GHz) channels.
Using DFB laser arrays has some advantages over alternatives such as tunable vertical cavity surface emitting laser (VCSELs), grating assisted codirectional coupler with sampled rear reflector (GCSR) lasers, and/or tunable distributed Bragg reflector (T-DBR) lasers. The advantages include higher power outputs, manufacturing complexity that is similar to conventional single DFB laser fabrication, wavelength stability, and the reliability and processing of DFB lasers.
When combining the outputs of a DFB laser array on-chip, additional circuits such as active-passive transitions, 1:N couplers, and integrated semiconductor optical amplifiers (SOAs) are required to compensate for the losses of the combiner. Placing the DFB lasers in a row along a single waveguide can eliminate the losses of the combiner. However, this approach introduces feedback and coupling problems in the longitudinal DFB laser array. Both combined and longitudinal DFB laser arrays also have limited scalability. The power losses in the combiner and device-to-device coupling limits the DFB laser array size to approximately 4-5 lasers and the total tunability to approximately 15 nm. This bandwidth is not sufficient enough to provide total c bandwidth coverage, which limits the DFB laser arrays to partial-band coverage.
An improved long-haul data light source preferably provides full c bandwidth coverage and has the cost, reliability and ease of manufacture of a fixed wavelength DFB laser. Cost considerations deter the use of complicated chips (such as GCSRs) or unconventional packages (such as a tunable VCSEL). In addition to chip manufacturing costs, the complexity of sophisticated control algorithms for GCSRs, VCSELs, and T-DBRs further increases the total cost of these devices.
A wavelength tunable laser according to the present invention includes a distributed feedback (DFB) laser array. The DFB laser array includes a first DFB laser diode that generates a first beam of light in a first wavelength range and a second DFB laser diode that generates a second beam of light in a second wavelength range. A microelectromechanical (MEMS) optical element adjusts to selectively couple one of the first and the second beams of light from the DFB laser array into an optical waveguide.
In other features of the present invention, the MEMS optical element includes a collimating lens and a MEMS actuator. The MEMS actuator adjusts a position of the collimating lens to select one of the first and the second beams of light. The MEMS actuator is preferably an electrostatic or a thermal actuator.
In yet other features, a focusing lens is located between the collimating lens and the optical waveguide. The optical waveguide is preferably an optical fiber suitable for telecommunications.
In still other features, the MEMS actuator includes an electrostatic comb drive structure, a flexible spring structure, and a drive circuit. The drive circuit actuates the electrostatic comb drive structure and the flexible spring structure to adjust the position of the collimating lens. Alternately, the MEMS actuator includes a thermal actuating structure and a drive circuit that powers the thermal actuating structure to adjust the position of the collimating lens.
In other features, large changes in the output wavelength are realized by activating different DFB lasers in the DFB laser array. Fine-tuning is preferably achieved by temperature tuning. The DFB laser array and the optical waveguide are mounted on a submount. A temperature of the submount is controlled by a thermoelectric cooler. The wavelength of the transmitter is adjusted by varying the current to the thermoelectric cooler.
In other features, the optical system further includes a beam splitter that reflects a first portion of one of the first and second beams of light and that passes a second portion of one of the first and second beams of light. A wavelength locker receives one of the first and second portions from the beam splitter and generates a wavelength measurement signal. A temperature tuning circuit receives the wavelength measurement signal and adjusts a temperature of the DFB laser array to vary the wavelength that is output by the DFB laser array.
In other features, a third DFB laser diode generates a third beam of light in a third wavelength range. The third wavelength range overlaps one of the first and second wavelength ranges. The third DFB laser diode is used to increase chip yield by providing redundancy.
In other features, a field lens is located between the DFB laser array and the collimating lens to remove vignetting effects. An optical isolator and a modulator are located between the beam splitter and the optical waveguide.
In still other features, the MEMS optical coupling system includes a MEMS actuator that tilts a mirror to select one of the first and second beams of light. The mirror tilts in first and second axial directions to compensate for misalignment of the collimating lens and the first and second laser diodes relative to an alignment axis.
Further features and areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.